1. Field of the Invention
The present invention relates to a voltage regulator that operates so as to keep an output voltage constant.
2. Description of the Related Art
In a technology for a related art voltage regulator, as illustrated in FIG. 9, an output voltage of a reference voltage circuit 21 and a voltage determined by dividing a voltage of an output terminal by a voltage divider resistor 51 are compared with each other by a voltage amplifier circuit 31 to control a PMOS transistor 41. For the purpose of obtaining a stable output voltage with respect to a power fluctuation, there is a need to allow a current to always flow regardless of a power fluctuation level (for example, refer to JP 2001-282371 A). Further, a phase of the entire system is compensated by a phase compensation circuit 61. The phase compensation circuit 61 includes a phase compensation capacitor 61a and a phase compensation resistor 61b (for example, refer to JP 2005-215897 A). The phase of the entire system is easily compensated by the phase compensation circuit 61, but the transient characteristic is deteriorated.
In general, in order to improve a response of the voltage regulator, a current consumption of the voltage amplifier circuit 31 needs to be increased. Therefore, the current consumption may not be reduced in the related art voltage regulator.
Further, in the phase compensation circuit 61 of the voltage regulator, a resistance value of the phase compensation resistor 61b may be set to be larger for the stable operation of the voltage regulator. As the output voltage of the voltage regulator changes, the output voltage of the voltage amplifier circuit 31 also changes. In a transient state where the output voltage of the voltage amplifier circuit 31 changes, when the resistance value of the phase compensation resistor 61b is large, it takes time to charge or discharge the gate of the output transistor 41.
FIGS. 10A and 10B are diagrams illustrating an input voltage and an output voltage of the phase compensation circuit in the related art voltage regulator, respectively. When an input voltage V1 of the phase compensation circuit 61 changes as illustrated in FIG. 10A, an output voltage V2 of the phase compensation circuit 61 changes as illustrated in FIG. 10B. When the resistance value of the phase compensation resistor 61b is small, the output voltage V2 is changed as indicated by a dotted line of FIG. 10B. On the other hand, when the resistance value of the phase compensation resistor 61b is large, the output voltage V2 is changed as indicated by a solid line of FIG. 10B. That is, there arises such a problem that the transient response characteristic is deteriorated by the phase compensation circuit 61, and the transient response characteristic of the voltage regulator is deteriorated.